Various regulations have been in place for years to limit certain emissions from internal combustion engines, notably compression ignition diesel engines. Engineers have devised many different aftertreatment mechanisms for diesel engines which reduce undesired emissions such as nitrogen oxides, unburned hydrocarbons, carbon monoxide, and particulate matter. Engineers have also known for many years that various aspects of engine operation could be varied to influence the chemical composition of engine exhaust rather than simply treating the exhaust once generated. One example relates to the insight that relatively hotter in-cylinder temperatures tend to be associated with reduced emissions of particulate matter, notably soot. High in-cylinder temperatures tend to promote the oxidation of soot particles within the engine cylinders. Hotter temperatures, however, also tend to be associated with increased emissions of undesired nitrogen oxides or “NOx” and may stress engine hardware. Tradeoffs such as this have limited certain operating methodologies from reaching their full theoretical potential.
Many compression ignition diesel engines are equipped with exhaust particulate filters. While generally effective at preventing soot emissions, such filters add significant cost to the engine system, and may also cause performance degradation due to high back pressure and/or fuel consumption penalties due to the need to regenerate such filters periodically. As an effort to reduce the frequency with which exhaust particulate filters need to be regenerated, combustion scientists have developed a variety of strategies for increasing the tendency towards oxidizing soot within cylinders of the engine without having other, unwanted effects on the engine or its emissions. Various engine geometries and operating patterns have been proposed to enhance mixing of gaseous combustion products within the cylinder to burn up as much soot as possible. Other strategies utilize fuel additives. U.S. Pat. No. 7,201,135 to Vachon utilizes a combination of factors such as bore size, injector spray orifice size, and injection pressure to achieve low smoke output in a power dense diesel engine. Despite advances in this actively researched field, there remains ample room for improvement.